1. Field of the Invention
The present invention relates to an optical pick-up apparatus with two light sources. More specifically, the present invention relates to an optical pick-up apparatus capable of utilizing two respective light sources, each having a different wavelength, for reading two respective types of optical discs, and wherein, no photo detector adjustment is required in order to read each type of disk.
2. Description of the Related Art
FIG. 5 shows the general structure of a conventional optical pick-up apparatus. A light ray radiating from a laser diode 11 is collimated by a collimator lens 12 and its beam form is shaped by a beam shaping prism 13. It passes through a beam splitter 14 and is deflected 90 degrees by a deflective prism 15. It is then focused by an objective lens 16 and radiated on an optical disc 17 as a minute optical spot. Recording , reproducing and erasing information on the optical disc 17 are carried out by this optical spot.
A light ray reflected off of the optical disc 17 is collimated by the objective lens 16 again, and its path is deflected 90 degrees by the deflective prism 15. It is reflected by the beam splitter 14 and is focused by a focusing lens 18. Cylindrical lens 19 provides astigmatism and the light ray is received on photo detector 20. It is photo detector 20 that detects the information signal and the servo signal as used within the optical pick-up apparatus.
Recent attempts to increase optical disc capacity have led to the practice of shortening the wavelength of the light source used to read these higher capacity discs. In general, the illuminated spot radial on an optical disc is proportional to the wavelength .lambda. of the light source used to read the disc, and the capacity of the optical disc is inversely proportional to the square of the wavelength .lambda.. Notwithstanding the trend toward using shorter wavelengths, there do exist optical disc drives that depend on the longer wavelengths. For example, the disc drive might depend on a reflective rate of the optical disc and also on the recording power. In such instances, it is impossible to reproduce and record information on a conventional disc by using a light source with a shortened wavelength.
Accordingly, for the purpose of establishing compatibility between conventional discs and optical discs has larger capacities, an optical disc drive might have two different kinds of light sources. One light source has a short wavelength (e.g., 650 nm), the other light source has a conventional wavelength (e.g., 785 nm). The simplest way to realize such a combined disc drive is to employ two separate pick-up apparatuses, each of which employs a light source having a different wavelength. However, in this case, such a drive would become too large and too expensive to be practical.
On the other hand, both the size and cost of such a combined arrangement could be reduced if two separate light sources, each having a different wavelength, could be processed using one common optical arrangement.
FIG. 6 shows such a conventional optical pick-up apparatus. The optical parts depicted in FIG. 6 are common to those optical parts depicted within the optical pick-up device of FIG. 5, the only difference being the light source 21. For example, a first light source comprising a laser diode (LD) chip having a wavelength of 650 nm and a second light source comprising a laserdiode (LD) chip having a wavelength of 785 nm are separated by a very small distance which ranges from scores to hundreds of nanometers (nm). One light source, in the light source portion 21, is located on the optical axis of the collimator lens 12, a ray from this light source travels as a solid line of FIG. 6. The other light source, in the light source portion 21, is located such that it is slightly departed from the optical axis of the collimator lens 12, a ray from this light source travels as a dotted line of FIG. 6. These two light sources are used selectively.
For example, an optical information recording and reproducing apparatus, as disclosed in Japanese unexamined patent (KOKAI) No. 06-259804, comprises a first light source, a second light source, an optical beam composing means for composing rays from respective light sources on the same optical path, an optical arrangement which makes a beam from the first light source focused on the first optical disc and makes a beam from the second light source focused on the second optical disc, and photo detectors for receiving reflective rays from both the first optical disc and the second optical disc.
In the FIG. 6 optical pick-up apparatus, one light source, in the light source portion 21, is located on the optical axis of the collimator lens 12, the other light source in the light source portion 21, is located such that it is slightly departed from the optical axis of the collimator lens 12. A light ray radiating from the collimator lens 12, which is departed from its optical axis, is incident to the inclining of that light ray from the objective lens. Furthermore, since aberration occurs and it is difficult to form a good optical illuminated spot on the optical disc 17, and also, since two rays having different wavelengths reflected on the optical disc are incident to the converging lens 18 with different angles, the incident respective positions of the reflected light rays on the photo detector 20 are different. Therefore, when adjustments are made to the photo detector 20 for a light ray having one wavelength, a servo signal detected by the photo detector for a ray having a different wavelength necessarily has an offset component.
Thus, there exists a need for an apparatus and method which allows for the use of a single optical arrangement for utilizing light rays of differing wavelengths to read optical discs of different respective capacities, and wherein optical signals received by the photo detector as reflected from the respective optical discs do not contain an offset component.